[Timothy]:
>The book "Hart's Rules for Compositors and Readers at the University
>Press Oxford" gives similar rules for German. It also mentions that
>ffi and ffl ligatures should never be used in German.

[Alain]:
...unlike French œ (oe), mandatory, stricto sensu, for correct spelling of
a word like "œuvre", but prohibited for a word like "coexistence" or
"Groenland" (note that there is no diaresis in the latter French name of
this big island whose Danish original name is Grønland, contrarily to what
is found in "Noël").

In fact it is exceptional to find a diaeresis on the e of unligatured
<oe>'s in French (the only other example I know is the first name
Joël[Joëlle]), and the French ligature, which is called by Grevisse a
"digramme soudé" (joined digraph), as pointed out a while ago by Michel
Suignard, from Microsoft (I adopted this term and I spread it, thanks to
him), serves as an anti-diaeresis, as I demonstraed in ISO/IEC JTC1/SC2/WG3
at the end of the 1980's. If it did not exist, a reform of the French
language would be necessary to put a diaresis on all unligatured <oe>'s
which are prononunced as two disctincly separate letters. The ligatured
<oe> is always a single pure vowel in French, although it is not always the
same sound (it can be roughly equal to German ö, to French e, or to French é).